Glucose 6-phosphate dehydrogenase is an enzyme catalyzing a reaction of, in the presence of a coenzyme nicotinamide adenine dinucleotide (hereinafter referred to simply as NAD) or nicotinamide adenine dinucleotide phosphate (hereinafter referred to simply as NADP), oxidizing glucose 6-phosphate into glucono-.delta.-lactone 6-phosphate and reduced nicotinamide adenine dinucleotide (hereinafter referred to simply as NADH) or reduced nicotinamide adenine dinucleotide phosphate (hereinafter referred to simply as NADPH) or vice versa. This enzyme is widely distributed in the natural world. Although glucose 6-phosphate dehydrogenase has been used in determining glucose-related substances and coenzymes or measuring the activities of glucose-related enzymes in clinical specimens using this function, it lacks stability for practical applications.
Various procedures have been described to improve the stability of glucose 6-phosphate dehydrogenase such as using an enzyme originating in a thermophilic bacterium (see JP-A-56-169598 corresponding to U.S. Pat. No. 4,438,199 and JP-A-59-151899 corresponding to EP-A-119 722, the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Satisfactory stability is not reliably achieved by these methods.
It has also suggested to stabilize glucose 6-phosphate dehydrogenase with a chelating agent or an SH compound in a reagent for assaying glucose (see EP-A-253 520), however satisfactory stability is not achieved in a weakly acidic region.